This invention relates generally to test instrumentation, and in particular to techniques for detecting bridge tap and other types of fault in a transmission line using frequency domain analysis.
Transmission lines are ubiquitous, and are use for transmission of communications and other electromagnetic signals. Examples of transmission lines include telephone wires, power lines, coaxial cables, twisted wire pair, and others. Typically, before a transmission line is activated for use, testing is performed to qualify the line. Such testing often includes the detection of faults (or events) corresponding to discontinuities in the impedance of the transmission line.
A common type of fault in a transmission line is a bridge tap. A bridge tap is a popular mechanism for attaching additional circuits to a transmission line, and comprises an additional line (also referred to as a lateral) coupled or spliced to the main line. The presence of the bridge tap affects the characteristic impedance of the transmission line to which it couples. The change in the line impedance due to the bridge tap is dependent on many factors, such as the length of the bridge tap, the circuitry coupled to the bridge tap (i.e., the loading), the characteristic impedance of the bridge tap, and other factors.
A transmission test set can be used to detect faults in a transmission line. More specifically, a time domain reflectometer (TDR) is conventionally used to detect discontinuities in the line impedance by transmitting pulses of energy, measuring the reflected pulses (if any), and determining the type and location of the fault(s) by analyzing the time-domain "signature" of the reflected pulses. The reflections are caused by both expected faults (e.g., gauge changes, splices) and unexpected faults (e.g., shorts, opens, water) in the transmission line.
The magnitude and phase of the reflected pulses are determined by the characteristics of the particular faults along the transmission line. For example, if the fault is an open circuit, the reflected pulse is in-phase with the transmitted pulse (i.e., both pulses have the same polarity). Alternatively, if the fault is a short circuit, the reflected pulse is out-of-phase with the transmitted pulse (i.e., the pulses have opposite polarities). Thus, by analyzing the magnitude and phase of the reflected pulses, an estimate can be made as to the identity and location of the faults in the transmission line.
The testing of a transmission line for faults is made challenging by a number of additional factors. For example, a transmitted signal in a transmission line naturally exhibits attenuation (or loss) due to the resistive, inductive, and capacitive characteristics of the transmission line. This natural attenuation degrades and distorts the transmitted and reflected signals, thus making it more difficult to accurately identify faults. The transmission line loss, particularly for a twisted wire pair, is also worse at higher frequencies, which tends to mask details in the reflected pulses.
From the above, techniques that can accurately detect the presence of faults, such as a bridge tap, in a transmission line is needed in the art.